Automatic telephone exchange system



7 Sheets-Sheet l E. P. G. WRIGHTk Filed July 12, 1929 AUTOMATICTELEPHONE EXCHANGE SYSTEM May 10, 1932.

ATTORNEY May 10, 1932.

E. P. G. WRIGHT AUTOMATIC TELEPHONE EXCHANGE SYS'GEM Filed July 12, 1929"7' Sheets-sheet 2 ESMOND P G WRIGHT INVENTOR ATTORNEY May 10, 1932.

E. P. G. WRIGHT AUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed July 12. 19297 Sheets-Sheet 3 NLL. EL.

ESMOND P. G. WRIGHT INVENTOR ATTORNEY May 10, 1932 E. P. G. WRIGHTAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed July l2. 1929 7 Sheets$heet 4`Esa/mln P. G. WRIGHT INVENTOR ATTORNEY May l0, 1932- E. P. G. WRIGHTAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed July l2, 1929 7 Sheets-Sheet 5ESMOND F? G. WRIGHT 'AT TORNEY May 10, 1932. E. P. G. WRIGHT 1,858,059 vAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed July l2, 1929 7 Sheets-Sheet 6BMZ BMI

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BTZ ATI BT l A412 I ESMOND F! G. WRIGHT INVENTOR ATTORNEY May l0, 1932.E. P. G. WRIGHT AUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed July 12, 19297 Sheets-Sheet 7 H .NO

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ESMOND P G. WRIGHT ATTOR NEY Patented May 10, 1932 Uniti-:D

ESMOND PHILIP GOODWIN WRIGHT, OF ALD'WYC-I, LONDON, ENGLAND, ASSIGNOR'IO INTERNATIONAL STANDARD ELECTRIC CORFORATION, OF NEW YORK, N. Y.

AUTOMATIC TELEPHONE EXCHANGE SYSTEM Application filed July 12, 1929,Serial No. 377,726, and in Great Britain September 13, 1928.

This invention relates to automatic or semi-automatic telephone exchangesystems. One of the objects of the invention is to provide a system inwhich all the switches used in the speaking route are single motionswitches of the 11o-normal type. A further obj ect of the invention isto reduce to a minimum the apparatus individual to the conversationswitches and to locate as much of the' 2G controlling apparatus aspossible in common circuits.

Referring to the drawings The reference characters which designate therelays are arranged with a reference letter l5 and a numeral (F/Q).

The letter designates the relay and the numeral the sets of contacts onthe relays. For illustration ESA/2 means that relay SA has two sets ofcontacts. F /9 means relay F 2G has 9 contacts. In referring to a relaythe letter will be alone used. Relay F /9 in the description will bereferred to as relay F and so on. The numeral simply shows the number ofcontacts operated by the relay.

Fig. 1 shows a preselector circuit comprising a primary nder LFl, asecondary finder LF2 and a distributor switch D.

Fig. 2 shows a selecting circuit at the first numerical switching stage.A first conversa.- tion group nder is indicated above the broken linewhilst below this line is illustrated a control circuit common to agroup of finder switches. The control circuit comprises a control switchR1, a. bye-pass switch R2 and an associated group of relays. Thecontroly switch R1 is equipped with wipers and banks 2 to 8 whilst thebye-pass switch R2 is equipped with wipers and banks C12, M12, M22, T12,T22, -1, -2, +1, +2, N21

0 and N 22. Certain groups of positions of the switch R1 are used foreffecting a marking in the bank of the bye-.pass switch R2, contacts inthe bank 7 in these positions being suitably strapped to the terminalsin the control bank C12 of the switch R2, whilst other groups ofpositions are used for operatively connecting the control circuit with aparticular nder switch over the banks 2, 4, 5, 6, and

7. Bye-pass conductors extend 4from the 5 terminals of the banks -1, 2,+1, +2 of the switch R2 to control circuits at the succeeding switchingstage, and bye-pass test wires extend from the terminals of the banksT1, T2 to finder switches at the succeeding switching stage. Theterminals in the marking banks M12, M22 are multiplied to correspondingterminals in the marking banks M of all the finder switches served bythe control circuit. The control circuit as shown is arranged to absorba iirst digit 6. Such a provision is desirable in circumstanceswellknown in the art and which it has not been thought necessary toenter into. For the purpose of this provision, however, two groups ofterminals of the bank 7 of the switch R1 are strapped to the terminalsof the control bank C12. Normally the first group is used for marking inthe bank C, but if the digit 6 which is to be absorbed is dialled theswitch R1 advances to the beginning of r the second group of terminalswithout making an effective marking, but in response to a further digita marking is effected over a selected terminal of the second group.

Fig. 3 shows a selecting circuit at the second switching stage. A secondgroup finder is shown above the broken line, whilst below this line isindicated a control circuit comprising a control switch R1 equipped withwipers and banks 1, 2, 4, 5, 6, 7, 8, 9 and 10 and a bye-pass switch R2equipped with wipers and banks C13, M23, M13, T23, T13, +2, 1, 2, +1 andN2. The banks or these switches are arranged similarly to those of theswitches Rl, R2 of the control circuit illustrated in F ig. 2 with theexception that the digit absorbing feature is omitted.

Fig. 4 shows a selecting circuit at the third or penultimate switchingstage. A penultimate group finder is indicated above the broken line andbelow this line is shown a control circuit comprising a control switchR1 equipped with wipers and banks 1,2, 4, 5, 6, 7, 8, 9 and 10 and abye-pass switch R2 with wipers and banks C14, M24, M14, GM, T24, T14,+2, 1, N21 and N22. At this switching stage single bye-pass conductorsonly extend to control circuits at the final switching stage. vThe banksof the switches R1, R2 are arranged in similar fashion to those of thecorresponding switches in Figs. 2 and 3. The terminals of the bank OMare connected to overflow metering devices.

Fig 5 shows a linal selecting circuit. A Linal iinder is indicated abovethe broken line and below this line is shown a control circuitcomprising a control switch Rl with wipers and banks 1 9, and a bye-passswitch R2 with wipers and banks X2, X1, M25, M15, T25, T15, C and N2.rFerminals oi the banks 7, 8, 9 in certain positionsl of the switch Rlare used for control purposes, whilst terminals of the banks l, 2, 3, 4,5' and 9 in other positions are used for associating the control circuitwith a particular iinal finder. Banks X1, X2 of the switch R2 are usedfor F. B. X. discrimination. Terminals in the banks T15, T are multipledto corresponding terminals in the {ind-er banks H and terminals in thebanks M15, M25 to corresponding terminals in the iinder banks M.

Fig. 6 shows a modified 'form ot selector circuit adapted to give accessto 20() lines and illustrates above the broken line a converse tionselector circuit comprising` two associated single-motion non-homingiinder switches A6 and B6. The switch A6 is equipped with wipers -A, +A,HA, MA and the switch B6 with wipers B, l B, HB, MB. Below the brokenline is shown a control circuit common to a group of conversationselector circuits and comprising three switches. a switch Rl the banksof which are indicated 1 10, a switch R2 the banks of which are in-(nested no, atie, Ain, irre, irri, iii-2,

A-tl, A-2 and A-L and a switch B3 the banks of which are indicated BC,BM2, BM1,BT2,BTl,B+2,B+l,B-2 andB-l. The switches R2 and are eachequipped with two sets of wipers. Access is had to byepass conductorsextending to the next switching stage over banks AT2, AT1, kd-2, .Pr-t1,

AL -2, Al of the switch R2 and over banks BT2, Bll, B-t2, B-l-l, B--27B-l of the switch R3. f

Fig. 6a, is a diagram showing the arrangement of the marking wiresbetween the control switches R2, R3. andV the conversation switches A6,B6. Marking wires extend from the banks BM2, BM'l to the control bank MBor the switch B6 and from the banks AM2, AMl to the control bank of theswitch A6. Group marking wires also eiltend from the first positions inthe bank ot the switch Bl' to the control banks LC and BC of theswitches R2 and R3, respectively. The later positions oi' the switch R1are used for operatively connecting the control circuit to an" one ofits associated conversation selector circuits over the banks l to l0.

Fig. 7 illustrates a method whereby a group of conversation switches maybe switched trom a faulty control circuit to an adjacent control circuitwhich normallyY serves a different group of conversation switches.

Fig. 8 is a schematic diagram illustratingI the general layout of atypical system.

The finder switches comprise 100 point single motion step-by-stepswitches of the ilo-normal type having two sets of wipers arranged 1800apart each wiping an arc of contact sets during one complete revolutionof the switch sha-ft.

The switches R1 are 50-point step-by-step single motion switches. As insome cases less than halt of the number of positions in the bank of Rlis required for marking purposes and for connecting the control circuitto the associated selector switches, the bank may 'nave a second normalposition, and have two corresponding groups of positions in the firstand second halves ot the bank, corresponding contacts being connected inparallel 'or the same marking or connecting function.

If the capacity of the R1 switch permits it a further similar group orgroups ot marking and connecting positions may be provided around thebank.

The switches B2 comprise 100 point single motion step-by-step switcheshaving two sets of wipers arranged to wipe simultaneously overcorresponding sets of bank levels.

The references used t'or referring to the various switches have forconvenience been attached in the drawings to the stepping magnets of thecorresponding switches.

A brief general indication oif the operation oi the system will now begiven with reference to Fig. 8.

A calling subscriber S becomes connected over a line circuit LC and irstand second line finders LFl, LF2 controlled by a distributor D to a freeirst group iinder S associated with an idle control circuit C. rhe virstdigit dialled sets the control switch Rl which marks the wanted group inthe bank of the bye-pass switch R2, which commences to advanceimmediately the impulses start arriving. The bye-pass switch R2 searchesfor an idle second group finder Si having an idle associated controlcircuit C1 and at the same time the switch R1 hunts for the first groupfinder S which is to replace the byepass. The group finder S is then seton the trunk selected by the bye-pass switch R2. Meanwhile a seconddigit is passing over the bye-pass switch R2 to the control circuit Clat'the second switching stage, which operates in similar manner to theiirst stage selecting circuit. The control circuit C at the first stageis released when the control circuit C1 at the second stage has becomeoperatively connected to the predetermined finder switch yS1 over thewipers ot' the R1 switch at that stage. The finder switch Sl is thenadvanced to the trunk selected by the R2 switch associated with thecontrol circuit Cl. In a similar manner Vthe connection is extended overa third group iinder S2 to a nal selectllt) ing stageunder the controlof a control circuit C2. At the final selecting stage the R1 switch ofthe selected control circuit C3 responds tothe tens and units digits andmarks a wanted line S4 in the bank of the R2 switch. vWhen the wantedline is engaged by the R2 switch, the R1 switch associates the controlcircuit C3 with the selected final finder S3 and a test is made ot' thecondition otthe wanted line in the control circuit C3. lf the wantedline is busy, a busy signal is given, whilst it the line is idle thefinder S8 is advanced to it and ringing current is applied. The controlcircuit C2- is released when the R1 switch of the control circuit C3 haspicked up the final nder S3. Vrlhe release ot the control circuit C2brings about the release of thecontrol circuit C3.

Referring now to Fig. 1 of the drawings a subscribers line terminates atthe exchange in the banks ot primary finders, such as LF, and also inthe banks of the connector switches, which are not shown. Thesubscribers lines are formed into groups, and there may be as many linesin a group as there are sets of contacts in the banks of the iindersLF1, say 100, there being a sulicient number ot' finders provided foreach group of lines to handle the traliic from that group. For eachgroup ot subscribers lines a common start relay SA is provided.

The secondary finders LF2 are linked to first group nders and aredivided into groups of sufficient size to handle the traliic which willbe directed through them. Each of these groups of secondary finders isarranged to search for a different primary linderin each of the primarygroups. Associated with each group of secondary finders is an allottingand controlling switch D which preselects an idle secondary finderlinked with an idle first group finder the control circuit associatedwith which also is idle. Preferably therefore the number of allottingswitches D is equal to the number of groups of secondary finders and tothe number of primary finders in each group.

lVhen a subscriber calls, a starting relay SA common to the subscribersgroup marks all the primary nders LFl serving this group and starts aset of secondary linders LF2 preselected by the allotting switches D.These secondary finders LF2 hunt until two have reached marked primaryfinders LF] and the two engaged primary finders then hunt until onefinds the calling line.

As an example, the originating trailic is assumed to be sul'cient tonecessitate 12 primary linders per group, and in consequence there areprovided 12 allotting switches and 12 groups of secondary finders. Thefirst group of secondary finders will have access to the first primaryfinders of each group of primary finders, the second group of secondarynders will have access to the second primary finders of each group ofprimary linders and so on to the twelfth group of secondary .finderswhich will have access to the twelfth primary finder in each primarygroup. l/"Vith this arrangement it will be seen that the tracing back ofcalls from the rst selector to the calling line is a simple matter, asthe positioning of the secondary finder will indicate the primary groupand the secondary group number will indicate the primary switch in usein that primary group. The positioning of the primary switch in use willindicate the subscribers line to which the first selector is connected.

A call will now be followed through in greater detail with reference toFigures 1 to 5 of the drawings.

TWhen a call is originated the line relay L is energized over thecalling .subscribers loop, causing the line to test busy to the finalswitches and placing a selectable potential on the test terminal H inthe banks of the primary nders LFl via both windings of the cut-oitrelay KA and contacts L2 operated. Relay L also closes a circuit for thecommon group relay SA which closes a circuit via SA2, f

HAQ, F1, K1 for the start relay J of all the idle distributor switches Dand places a potential via the windings of relay HA and contacts SAl onthe marking Contact C of all the primary finders such as LFl serving theline group to which the calling line belongs.

Relay J oi" each distributor switch D completes the following circuitcontacts B2, J3, winding ot relay lLvdistributor wiper DT, wire T,contacts HS2, Figure 2, B2, P1, Q1, E10, wipers of switches R1 and R2 intheir home positions, contacts B7 to battery.

Relay H closes a circuit via El, F8, K7, H3 wiper F21 and interruptersprings of a switch magnet (similarly designated in other parts of thedrawings) for the stepping magnet F22 of the associated secondary finderLF2. At the same time a circuit is prepared for the test relay E overcontacts H1, K5, wiper F2T or" the distributor switch D and wiper C ot'the secondary finder LF2. A spark quenching condenser is connected tothe magnet F 22 over wiper SQ2.

The secondary finder LF2 steps until it reaches a marked primary finderLF1 thereby causing the response ot relay E which opens the circuit ofthe driving magnet F22 at contacts El and closes at E2 a circuit via HE2for relay'F which locks via contacts F9, .l2 and prepares a circuit fortest relay B at contacts F42. y y

rlhe relay HA will not operate in series with a single relay E, but whena secondary nder controlled from another distributor switch also engagesa marked primary iinder there will be two E relays in parallel and relayHA will operate and open at contacts HAl the circuit to the markingcontacts via its non-inductive winding and battery thereby preventingother E relays Jfrom operating.

lt a primary finder be already engaged when a secondary finder passes onto it, a relay HE is energized over contacts K6, H2, distributor wiperMN, wiper and multiple Contact L of the secondary finder Ll4`2, multiplecontact such as L and wiper of the secondary tinder already engaging theprimary linder, trontcontacts such as KSl or HSl of the attaching orswitching relay KS or HS, Fig. 2 et the group tinder ot the engagingsecondary finder link, and the grounded hold wire H. Relay HE operatesand at contacts HE2 holds open the circuit of relay F and closes analternative circuit for the stepping magnet F22 independent of relay E,so that the secondary inder continues to advance. A primary linder willbe taken into use, however, it it is idle, even it it is standing on abusy line since in this case the multiple L will not be grounded.

It will be seen that the secondary finders continue to hunt until twohave seized primary finders ot the group to which the calling partybelo-ngs.

1When relay HA operates it opens at contacts Hl'l2 the starting circuitot the J relays of the various distributor switches but by this timerelays J of the distributor switches associated with secondary iinderswhich have seized marked primary tlnders have been held energized viacontacts Kl and F1.

The operation ot relay F also completes a circuit tor the stepping`magnet Fll ot the primary iinder Lll via contacts Bl, F2, distributorwiper F12 and tinder wiper B. rEhe common spark quenching condenser isnow conected to the magnet F22 via contacts F7 and wipers SQA, SQ.llfhen a. primary tinder engages the calling line relay B is energizedvia contacts Fa, wiper lllrl of the distributor switch D, wipers H otthe secondary and primary tinders, and the subscribcrs cut-oli' relayKA.

Relay B locks up over contacts F5, Bl and at contacts B2 opens thecircuit ot relay H. Relay B also opens the circuit of the steppingmagnet Ell at contacts Bl and closes a circuit via contacts Bl, F6 forrelay K u hich at contacts Kl opens the circuit of relay J, and atcontacts opens the circuit oll relay E. Relay l? now relapses and opensthe circuit of relay B at contacts F5. On the relapse ot relay l? relayl locks over the make etore break contacts F6 and contacts K3, Hl.

A. circuit is now completed from ground at contacts Hl, F3, K2,interruptor contacts and magnet D ot the distributor switch whichrotates to find an idle secondaryT link the associated finder and itscontrol circuit of which are also idle. When this occurs relay Henergizes over contacts B2, K9, wiper DT and the test wire T, and atcontacts Hl opens the stepping circuit and unlocks relay K which in turncle-energizes relay H.

Should the distributor switch engage a secondary link which althoughidle has been preselected by another distributor switch, the relay HE isenergized over contacts K6 the wiper CM, multiple contact, wiper such asCM of the other distributor to ground at contacts such as K4. Analternative ground is thereby provided for the relay K and magnet D viacontacts Kel, HEl and the distributor switch continues to step in spiteof the momentary response ot relay HE.

lt will be evident that it only one marked primary finder be idle thiswill hunt alone for the calling line. It is also possible for thecalling line to have been picked up by a primary tinder before a secondstarts its hunting operation.

As already described when a distributor is seized the selector circuitVwith its associated control circuit preselected thereby is tested overthe wire T, contacts HS2, B2, P1, Q1, Ell), the wiper and home contactof the bank 7 ot the switch Rl, the home contact and wiper ot bank C12ot' the switch R2, B7 to protective resistance and battery in order todetermine il' the control circuit is still idle. It the circuit is'found to be tree by the distributor circuit that circuit will'reducethe potential on the common point between HS2 and B2 and consequentlycause the control circuit to test busy. W hen the preselector circuithas completed connection to the calling subscriber, a ground potentialwill be placed on the wire K causing the operation of relay KS. Acircuit is now completed for relay A from battery and one winding ofrelay A., H3, KS2, negative wire, subscribers loop, positive wire Q53,H5, second winding of relay A to ground. Relay A completes the obviouscircuit Jor relay B over A2 and relay B opens the testing circuit at B2and closes a locking circuit for KS through B3 and KS. B7 opens thecircuit to the battery and energizes the relay lil. At the tirstinterruption relay l releases but relay B being ot the slow release typeremains operated and completes a circui=J for the magnet Rl through E4,B5, winding ot relay C and Al to ground. At the end of the interruptionrelay A reoperates and the switch Rl steps into position 2. ln position2 the circuit ot relay M is opened and when relay M releases a circuitis completed for the magnet R2 through H8, interrupted springs, Ml,wiper and home position or" the bank N21 of switch B2, C2, Q1, Pl, B3 toground. The switch B2 takes a step and continues stepping by means ofits own interrupter springs and the off normal contacts of the bank N21to ground at E6 until relay lvl reoperates. rlhe switch Rl takes onestep for each impulse received from the subscribers dial and thestrapping between the banks n group are engaged, the switch R2 willoperated 7 and C12 is so arranged that the circuit for relay M iscompleted whenever the R2 switch reaches the commencement of the groupcorresponding to that on which the R1 switch is standing. l/Vhen thedigit is completed relay C will release after a period and when relay Moperates due to the switch R2 having reached the position indicated bythe bank 7, relay M will reoperate, as already described. and a circuitwill then be completed for relay E from battery and winding of relay E,G4, V, C1, W74, M2, wip er and oit normal contacts ot the bank N21, E6to ground. Relay E closes its light-spring contact E9 and locks over itssecond winding to ground at B3. E10 opens the circuit for relay M whichreleases and recloses the driving circuit of the R2 switch through M1,wiper and ott' normal contacts ot the bank N21, E5, Q1, P1 to ground atB3. Contacts E12 and E13 close the testing' circuit for relays P and Qrespectively and one or both of these relays will operate if the outletson which the wipers are resting are tree. It relay P alone operates thedriving circuit of the switch will be opened at P1 and at P2' a circuitwill be closed tor relay H which will lock up via H6 to ground at B3. Itrelay Q operates the driving circuit ot the switch will be opened at Q1,and at Q2 a circuit will be closed for relay J which will operate andclose at J1 a circuit for relay H which will also operate. .H6 willprovide a locking circuit for both the relays H and J. It all theoutlets of the step eventually to the last contacts of the group inwhich position a circuit will be closed for the relay M through H10, B8,wiper and bank contact C12, E11, Q1, P1 and B3 to ground.

i Relay M will open the driving circuit of the switch R2 at M1, andclose at M2 a. circuit tor relay G through E8. Relay G is of the slow tooperate type and will not operate during the period when both relays Eand M are during the initial operation of the former. However, on thelast contacts of the group the circuit for relay G is maintained andthis relay will eventually operate and connect at G7 the busy tone tothe calling subscriber from lead BT.

^ relays T and H are operated pleted for the TWhen relay E operated acircuit was com pleted for the switch R1 through E3, the interruptersprings, and T1 to ground. This will cause the R1 switch to continuestepping until the relay T operates through wiper and bank contact 2,contacts KS to ground at B3.

t will be understood that there is only one group tinder accessible tothis control switch with its relay KS operated. lhen both the due to thefact that the R2 switch has found a free outlet in the required groupand the R1 switch has found the nder which it has been predeterminedw'ill complete the call, a circuit is comtinder magnet S through bankcontact and wiper 5 of switch R1, M3, H9 and T2 and at the same time aground. is eX- tended through T3, the wiper and bankV contact 7, to thefinder wiper M and when the finder wipers stand on the outletcorresponding to that on which the switch R2 has switched through, theground on the Ender wiper M will be extended to the control wiper M12 orM22, J4 or J5, Hllto relay M. The operation ot M opens the circuit forthe tinder magnet S at M3 and closes the circuit for the relayl G by E8,M2, the o' normal bank N21, E5, Q1, P1 to the ground on the test Wire.Relay G will operate and close the circuit for relay HS through bankcontact and wiper 4, G1, M4, H9 and T2 to ground. HS will operate andHSG will extend ground over the H wiper to the H wire ofthe succeedingfinder in order to mark that switch to the succeeding control. nContacts HS3 and HS4 will prepare the permanent circuit to replace thebye-path. HS5 grounds the H wire to hold the cut-oli1 relayindependently of the control circuit. The control circuit does notrelease until the ground potential is removed from the T12 or T22 wireby the succeeding control circuit. When the relay H operated the circuitfor relay A was opened and relay B released also after a short period.Relays E, KS and T where maintained, however, by the ground potential onthe test wire, which is'maintained by the succeedingcontrol circuitunder the control of the calling subscribers loop. lVhen the succeedingfinder removes ground from the T wire, relays T, KS and E, M and Hrelease. When relay H releases, the homing circuit for the switch R1 iscompleted through E4, B4, H7, o'l" normal contacts and wiper ot bank 3,interrupter springs T1, to ground. In a similar way the homing circuitfor the switch R2 is completed through H8, M1, wiper and ott normalcontacts N21, E6, to ground. When both these switches have returned totheir home positions, the battery is reconnected to the test wirethrough B7.

Should it happen that thesubscriber dials a predetermined digit whichthe control circuit has been arranged to absorb, the switch R1 will cometo rest in a position similar to position 6 in the drawings. A circuitis completed for relay W from battery and winding of relay W, wiper andbank-contact 'of bank 8 of the switch R1, B3 to ground. Relay W willoperate and lock through W7, C1 and W5 to ground. At W8 a particularcontact is earthed in the control bank C12, whereby when the tion relayM is energized and arrests this switch until the next digit is dialled.At the same time a circuit is complet-ed for the magnet R1 through C4,W2, the bank contact and wiper of the bank 3, the interrupter springsand T1 to ground, causing the switch R1 to step forward until it reachesthe first contact connected .to W1 where its circuit is bye-pass switchreaches this posiopened. The wiring ot the bank 3 is so arranged thatthe R1 switch will step automatically to the 11th position in readinessfor the second set of impulses. Then the second set of impulsescommences relay C operates, as already described, and thc circuit forrelay W is opened causing this relay to release. The subsequentoperation of relays E, P or Q, H and T will take place, as alreadydescribed.

Should it happen that the subscriber tails to dial, a circuit will becompleted from the pulse lead A through the home ContactV and Wiper ofthe bank 6, G3, B6, B7 to the relay G which will lock up over G2, C3 andB3 to ground. After some predetermined period a second pulse will bereceived over the pulse wire B, wiper and home contact ot bank N22 ofthe R2 switch,` G5 to relay E. Relay E on operating will lock through E9to ground at B3 and open the circuit tor relay G at E7. Relay P willoperate from ground at B3 through G9, H13, E12, wiper and home contactof the T12 bank to protective resistance and battery. Relay P will causethe operation of relay H and the subscribers loop will be switched viaH2 and H4, J6 and J8 through the home contacts ot the banks --1 and +1,G8 and E2, back to the relay A. When relay E operated, it closed thedriving circuit ot the magnet R1, as already described and this switchwill step forward until relay T operates. Relay T will close a circuit,as already described,

s for the tinder magnet S. When the tlnder has been driven to theterminals corresponding to those on which the switch R2 is standing (i.e. its home terminals), relay M will operate and a circuit will beclosed for relay G through E8, M2, wiper and home contact of the bankN21, M4, H9 and T2 to ground. G1 closes a circuit for the relay HS whichis maintained through HSG, H wiper and home contact of the nder S, homecontact and negative wiper, HS3, subscrihers loop, HS4, positive wiperand home contact, to ground. The operation ot G6 opens the circuit forrelay A which releases and opens the circuit for relay B. Relay B inturn opens the circuit for relays E, G. H and T causing the switch R1 toreturn to normal. The finder relay HS is maintained operated over thesubscribers loop until the subscriber releases. The same operationsoccur if the subscriber Jfails to dial a second digit after the switchR1 has gone to its 11th poy sition after responding to the digit 6.

ed through the Wire T, contacts HSl, TT3,

vllVhen the control circuitis first seized a dialling tone it requiredis introduced through E1 to the winding of relay A. l/Vhen the relay Eoperates the dialling tone is removed.

Referring now to Fig. 3 this selector circuit with its associatedcontrol circuit is test- TTG, P1, Q1, E11, the wiper and home contact otbank 9, of switch R1 the home contact and wiper of bank C13 of switch R2B5, to resistance and battery. It this resistance is connected to thetest wire, the previous control circuit will switch through extendingthe suhscribers loop to conductors and thereby, causing the operation ofrelay A through H2, H4. Relay A closes the obvious circuit for relay Bthrough A2. Relay B places ground on the test wire at B1 and removes thebattery from the test wire at B5. At B6 a circuit is closed tor relay Mfrom battery and winding ot relay M, H14, B6, wiper and home contact ofthe bank C13, home contact and wiper ot the bank 9, E11, Q1, P1, TT6,B1, t ground. At the first impulse, relay A releases but relay B beingof the slow release type, remains operated and completes a circuit forthe magnet R1 through E3, B2, winding of relay C and A1 to ground. Atthe end of the impulse relay A reoperates and the switch R1 steps intoposition 2. ln position 2 the circuit for relay M is opened by the wiper9 and when M releases a circuit is completed for the magnet R2, throughits interrupter springs, H7, M1, wiper and home position ot the bank N2of switch R2, G1 of B1, to ground. The switch R2 takes a step andcontinues stepping by means of its own interrupter springs and the ott'normal contacts of the bank N2, to ground at E until relay M reoperates.The switch R1 takes one step for each impulse received from thesubscribers dial and the strapping between the banks 9 and C13 is soarranged that the circuit for M is completed whenever the R2 switchireaches the commencement ot the group corresponding` to that on whichthe R1 switch is standing. When the digit is completed, relay C willrelease after a period and when relay M operates due to the switch R2having reached the position indicated by the bank 9, relay M willoperate as already described and a circuit will then be completed forrelay E from battery and winding ot relay E, G3, C3, M2, wiper and offnormal contacts of the bank N2, E5, to ground. Relay E will close itslight spring contact E10, and lock over its second winding to ground atB1. E11 opens the circuit for relay M which releases and recloses thedriving circuit ot the R2 switch through H7, M1, wiper and ott" normalcontacts of the bank N2, E4, Q1, P1, TT6, to ground at B1. Contacts E13and E14 close the testing circuits tor relays Q and P respectively, andonef or both ot these relays will operate it the outlets on which thewipers are resting are free. T relay P alone operates the drivingcircuit of the switch will be opened at P1 and at P2 a circuit will beclosed for relay H which will lock up via H5 through TTS to ground atB1. It relay Q operates a driving circuit of the switch will be openedat Q1, and at Q2 a circuit will be closed for relay J which will operateand close at J 1 a circuit for relay H which will also operate. H5 willprovide a locking circuit for both the relays H and J. The proper wiperset will be connected up at contacts J -J 8. It all the outlets of thegroup are engaged, the switch R2 will step eventually to the lastcontact of the group, in which position a circuit will be closed for therelay M through H14-, B6, wiper and bank contacts C13, E12. Q1, P1, TTG,and B1 to ground. Relay M will open the driving circuit of the switch R2at M1, and close at M2 a circuit for relay G through E7. Relay G is ofthe slow to operate type and will not operate during the period whenboth relays E and M are operated during the initial operation of theformer. However, on the last contacts of the group the circuit for relayG is maintainedand this relay will eventually operate and connect at G9the busy tone over wire T to the calling subscriber.

When relay E operated a circuit was coinpleted for the switch R1 throughE2, the interrupter springs and T1 to ground. This will cause the switchR1 to continue stepping until the relay T operates through wiper andbank contact 4, contacts HSl, and the hold wire H to the precedingfinder. It will be understood that there is no more than one linderaccessible to this control switch with a ground potential on the H wire.If this ground has not been applied when the switch R1 passes over thecontacts particular to the predetermined conversation switch, due to thesetting of the conversation switch at the preceding stage not beingcompleted, the switch R1 will repeat its hunt. lVhen relay T operatesthe obvious circuitis closed for the relief relay TT. TT1 and TT2 extendthe subscribers loop to relay A through the wipers and contacts of banks1 and 2 thereby replacing the bye-pass over conductors -land ContactsTT3 remove the ground from the test wire causing the release of theprevious control circuit. TT4 provides a locking circuit tor the relay Tand extends the ground to the hold wire to hold the previous switchingrelays. When both the relays TT and H are operated due to the fact thatthe switch R2 has found a free outlet in the required group and theswitch R1 has found the finder which it has been predetermined willcomplete the call, a circuit is completed for the linder magnet Sthrough bank contact and wiper 7, M3, TT?. H8 and at the saine time aground is extended through TTS and the wiper and bank contact 9, to thelinder wiper M and when the finder wipers stand on the outletcorresponding to that on which the switch R2 has switched through, theground on the finder wiper M will be extended over the control wiper M1or M2, Je of J5, H13, to relay M. The operation of M opens the circuitfor iindermagnet S at M3 and closes the circuit for the relay G via E7,M2, the ott-normal bank N2, Eet, Q1, P1, TT6 to the ground on the testwire. Relay G will operate and close the circuit for relay HS throughbank contact and wiper 6, G1, M4, TT7, H8, to ground.` Relay HS willoperate and HS6 will 'extend ground overthe H wiper to the H wire of thesucceeding linder in order to mark that switch to the succeedingcontrol. Contacts HS2 and HS3 will prepare the iermanent circuit toreplace the bye-path. HSl grounds the H wire to hold the previousswitches independently of the control circuit. The control circuit doesnot release until the ground potential is removed from the T wire by thesucceeding control circuit. When the relay H operated the circuit forrelay A was opened and relay B released also after a short period.Relays E and T were maintained, over J2, H9, E13, or J3, H12, E14.however, by the ground potential on the test wire which is maintained bythe succeeding control circuit under the control of the calling'subscribers loop. When the succeeding finder removes ground from the Twire, relays T and E release and subsequently relays TT, M and Hrelease. 7When relayH releases, the homing circut for the switch R1 iscompleted through E3, B3, H6, off normal contacts and wiper of bank 5,interrupter springs, T1, to ground. In a similar way the homing circuittor the switch R2 is completed through H7, M1, wiper and off normalcontacts N2, E5, to ground. Then both these switches have returned totheir home positions the battery is reconnected to the test wire throughB5.

Should it happen that the subscriber fails to dial, a circuit will becompleted from ground on the pulse lead A through the home Contact andwiper of the bank 8, E9, G5, B4, E6, to the relay G which will lookup toG4, C2, B1 to ground. Aft-er some predetermined period a second pulsewill be received over the pulse wire B wiper and home Contact of bank 10of the switch R2, G2, to relay E. Relay E on operating will lock throughE10 to ground at B1 and open the circuit for relay G at E6. Relay P B1,through G7, H11, E14, wiper and home Contact ot the T13 bank toprotective resistance and battery. Relay P will cause the operation ofrelay H and thel subscribers loo will be switched via H1 and H3, J 6 andJ8 through the home. contacts of the banks -1 and +1, G8 and E1, back tothe relay A. When relay E operated, it closed the driving circuit ot themagnet R1, as already described and this switch will step forward untilrelay T operates. Relay T will close a circuit for the relay TT, whichas already described closes a circuit for the finder magnet S. ifllhenthe finder has been driven to the terminals corresponding to those onwhich the l* 'n' will operate from, ground l switch B2 is standing (thatis the home terminals), relay M will operate and a circuit will beclosed for relay G through E7, M2, wiper and home contact of the bankN2, Ma, TTT and H8 to ground. Gl closes a circuit for the relay l'lSwhich is maintained through HSG, H wiper and home Contact of the finderS, home contact and negative wiper, HS2, subscribers loop, HSS. positivewiper and home contact, to ground. rllhe operation of G8 opens thecircuit for relay A which releases and opens the circuit for relay B.Belay B in turn opens the circuit for relays E, G, H and T causing theswitch B1 to return to normal. The iinder relay ltlS is maintainedoperated over the subscribers loop until the subscriber releases.

Referring now to the penultimate selector' circuit shown in Fig. 4 afinder with its associated control circuit is tested in a similar mannerto the second group selector circuit through BSl, GSS, TTS, T'l, P2, Q1,E12, wiper and home contact of the bank 10 ot the Bl switch, homecontact and wiper of the bank C14 ot the R2 switch. B6 to protectiveresistance and battery. It the nder and control circuit are found to befree, the preceding control circuit switches through the callingsubscriber by means ot the bye-pass negative and positive wires therebycompleting a circuit for relay A over H1. Relay A completes the obviouscircuit for relay B and a stepping circuit tor the magnet R1 similar tothat described for the absorbing'and group selector circuits. Thestepping of the switch R2 is completed through Ml, the wiper and homecontact of the bank N21, Cl and B1 to ground. lVhen on the completion ofdialling tbe switch B2 reaches the commencement of the group, thecircuit for relay E is completed through Gl, C3, M2, wiper and ott'normal contact of the bank N21, H5, Ea to ground. Relay E completes thetesting circuits for relays P and Q at ll and E15 respectively andshould a tree outlet be available relay P and Q will close the circuitsfor relays H and J, as alreday described. llhen relay E operated acircuit was completed for the magnet R1 through E2, interrupter springs,Tl to ground causing the switch B1 to step until it swiper l isassociated with the penultimate iinder which it has been predeterminedshall complete the connection. In this position a` ground potential onthe hold wire fed forward from the preceding linder is extended to relayT which operates and closes the obvious circuit for relay TT. lllhenboth relays TT and H are operated due te the fact that the switch B2 hasfound a free outlet in the required group and the switch B1 has foundthe finder A. circuit is completed for the linder magnet S through thecontact and wiper ot bank 8, Ml, E8, H6 and TTT to ground, and at thesaine time a ground is extended through TTS, the Wiper and contact ofbank 10, to the finder wiper M, and When the finder wipers stand on theoutlet corresponding to that Von which the switch R2 has switchedthrough the ground on the finder wiper M will be extended to the controlWiper Mil or M24, .la or J5, HS, to relay M and battery. The operationof relay M opens the circuit for the finder magnet S at Ml and places aground potential via contacts M3, the wiper and contact bank 7, BSS onthe H wiper of the penultimate finder. The penultimate control circuitremains in service While the last two digits are being received by theiin al selector circuit and the called line tested. After the linalselector circuit has completed the test, it removes ground from the testWiper T13 or T23 which was holding the penultimate control circuitcausing relay E to release. A circuit is now completed for relay GSthrough AS2, contact and wiper of bank 6, TT6, non-inductive resistance,E6, TT'?, to ground. rlhe non-inductive resistance is of such a valuethat the marginal relay BS will not operate in multiple with GS. It ispossible, however, that the final inder has placed a ground potential onthe H wiper of the enultimate finder in which case it will be ied backthrough BSS, contact and wiper ot bank 7, M3, E7, TTG, wiper and contactof bank 6 to relays GS and BS in which case both relays will operate.The ground potential will only be placed on the H wiper by the linaliinder when the called subscribers line is free as is explained later.The operation of relay GS will complete a circuit for relay AS via GSl,DS1, subscribers loop DS3, GS6. Relay A provides a holding ground forrelay GS; relay BS in operating completes the ringing circuit throughFS4, BS?, called subscribers loop BSt, FS2, the ringing tone battery RR.When the called subscriber answers, relay FS operates and completes acircuit for relay DS through FSI and FSS. When the relay DS operates itreverses the direction of current in the called subscrbers loop formetering purposes. Talking current is supplied to the calling and calledparties through relays AS and DS respectively. DS6 provides a lockingearth for relay BS; the calling subscriber on releasing causes the relayAS and subsequently the relay GS to release and at GSG remove the groundholding the preceding switches which are made to release. Thepenultimate finder will remain operated as long as the relay DS is heldby the called subscriber. Relay BS which is held by relay DS preventsthe penultimate finder being taken into use by opening contacts BSl. lfthe called subscriber releases first relay DS makes the circuit of relayBS dependent upon relay GS and the called subscriber is held until thecalling subscriber releases: during this time a signal is given via DSTand FSS to the supervisory zlarm over lead GSH. lf the calledsubscribers line is engaged and relay GS opcrates without relay BS thebusy tone lead BT is connected up at GSS and BS5 to the callingsubscriber. In either case GS5 opens the circuit for relay T which inturn opens the circuit for relay TT causing the release of the controlcircuit. If no outlets are found free by the control circuit, relay Gwill be operated over the following circuit B5, G2, E10, M2, wiper andoff normal contact of the bank N21, H5, E3, Q1, P2, TT'5, B1 to ground.Relay G will extend ground from B1 through G5 and E5, non-inductiveresistance, TTG, to wiper and contact of bank 6, to cause the operationof relay GS which will connect the busy tone circuit to the callingsubscriber, as already explained. If the subscriber fails to dial onseizing the penultimate control circuit, the ground on .the pulse wire Awill cause the operation of relay G through the home contact and wiperof bank 9, E9, G2, B5, to relay G. Subsequently the ground on the pulsewire B will operate relay E through the wiper and home contact of thebank N22, and G3. lilith relays G and E operated ground from B1 isextended over G5 and E5, noninductive resistance, TT6, wiper and contactof bank 6 to relay GS, applying'V a busy tone t-o the callingsubscriber. It will be seen that when relay H operates the negativewiper only is extended to the fina-l finder the ground for the impulseVcircuit being maintained in the penultimate finder. An additional bankOM on the switch R2 is provided for over-flow metering purposes and aground potential is placed on the Contact associated with the grouprequired by G8 if there are not outlets available.

Referring now to Fig. 5 a final nder with its associated control circuitis tested over the wire T, HS1, T8, wiper' and home contact of the bankC5 of the switch R2, the home contact and wiper 9 of the switch R1, V6,B7, protective resistance to battery. The negative wire is extended overthe bye-pass to relay A which operates and operates relay B over theobvious circuit. When the first impulse commences a circuit iscoinpleted for the magnet R1, via T2, E3, B1,

A2, to ground. At the same time a parallel circuit is completed throughB4 for the relay C. lVhen the switch R1 steps into position 2 a circuitis completed for the magnet R2 through its interrupter springs P2,

' G3, the Contact and wiper of bank 8 of the switch R1, H1, M2, G1, E2,B3 to ground. Vhen the switch R2 reaches the position marked by the bank9 of the switch R1, relay M will operate from ground at B5, over V6 andB8 causing the circuit of the magnet R to be opened at M2. Then thefirst digit is complete and relay C has released with relay M operated,a circuit is completed from ground, B3, E2, G1, M1, wiper and contact ofbank '.7 of the switch R1, Cl, G5,

to the relay G which operates. Relay locks over at G6 through E6 toground at B5.

Relay G completes a circuit for the magnet R, through G4, strappedcontacts in positions 2 to 11 and wiper of bank 7 M1, and interruptersprings to ground causing the switch R1 to take a step to its nextposition in which the circuit for relay M is opened and the drivingcircuit for magnet Rl replaced by G2, the strapped contacts in position2 to 11 and wiper of bank 8 of the switch R1, and H1 and M2. The switchR1 continues to be stepped by this circuit unitl it reaches position 12where it awaits the units digit. The stepping for the units digit issimlar to that descrbed for the tens ex cept that the switch R2 has nocircuit until relay C releases the second time. The switch R2 then stepsthrough P2, C2, contact and wiper ofrbank 8 of switch R1, H1, M2,intcrrupter springs of the switch R1 to ground until relay M operateswhen the R2 switch is standing on a position marked by the R1 switch.

Relay M completes a circuit through the wiper and contact of bank 7 tothe relay E which operates and locks through V1, E?, B5 to ground. RelayE opens the circuit for relay G at E6 but while G is releasing, thisrelay bein@ of the slow to release type, a testing circuit is completedfrom ground at H7, G9, TR1, E8, winding of relay P, J6 or J7 to the testwiper T25 or T15. Relay J is the discriminating relay and is operatedwhen the R1 switch steps for the tens digit on alternate contacts onlyof bank 1 of the switch R1 which are grounded. Tf the switch R1 is onone of' these alternate positions when relay G operates, relay J willlremain operated through G8, J1, T3, B5 to ground. If relay J is notoperated when relay G operates it will have no further opportunity. Ifthe line called is free relay P will operate and close the obviouscircuit for relay H. When relay G releases a circuit is completed forthe magnet R1, through T2, El, G1 and the interrupter springs to ground.This stepping will continue until the relay T operates through E5, thewiper and contact of bank 2, and HSS to the hold wire of the finalfinder which has been predetermined by the penultimate selector circuit.The ground at the interrupter springs of magnet R1 is then extendedthrough G1, E1, T1, H6 and M4 to the wiper and Contact of bank 4.- andfinal finder magnet S. At the same time relay M is extended through B8,V6, wiper and bank contact 9, to the marking wiper M of the finalfinder. l/Vhen the final finder wiper stands on the outlet correspondingto that on which the R2 switch is standing, relay M is extended to themarking wiper M or M15 of the switch R2, J l or J5, H8 to ground. RelayM opens at M4 the circuit for the final finder magnet Si and closes thecircuit at lal through wiper and contact of bank 3 tor relay HS whichlocks through ll-llL to the test wire which is grounded from B5 over H9,J6 or JT, and wiper T or T15. H51 opens the bye-path test which releasesthe penultimate control and HS2 places the ground potential on the Hwire causing the operation ot relay BS in the penultimate finder. li"the called subscribers line is engaged, relay P will not operate andwhen relay G releases the circuit for relay P is opened. The steppingcircuit for the magnet B1 is closed at G1, as already described, andwhen relay T operates grounf on the bye-path test wire is removed at T3; T4 provides a locking circuit for relay T. In this case there is noground on the hold wire H and when the penultimate control releasesrelay GS only will operate. Should it happen that the line tested is thelirst of a P. B. X. group and relay P does not eperate, a circuit iscompleted from battery and protective resistance on the contact of thebank X1 or X2, J2, or J3, B9, V2, winding of the relay V, G10, H5, B5 toground. Relay V locks at V3 over a protective resistance to battery andopens at V1 the locking circuit for relay E which releases and replacesthe circuit for relay G at E0. Tae

circuit of relay M is switched over by V5,

J2 or J3 to the P. B. X. discriminating' minals and the circuit for themagnet R2 i connected due to the 'tact that relay lll releases. At eachstep forward` a testing circuit is provided for relay P through *es teground at B5. 1V hen an idle line is engage-il relays P and H energizeand a circuit is closed over H10 `lor relay E, which opens the circuitof relay G and closes the circuit for driving the switch B1. The switchB1 trunks for the predetermined conversation switch and the operationcontinues as before. On the last line ot a group, iic relay P has notalready operated, ground from tl. discriminating bank X1 or X2 causestlie operation of relay M which opens the d i circuit of the magnet B2and causes the reoperation of relay E. Relay G will release after aperiod opening the circuit tor i lay V at G10. Due to the tact that relTT l not operated when relay T operates, signal is passed back to thefinder. Should it ha een that the subscr- .L 1 fails to dial a pulsecircuit is completen the two-ste rela i TP. via B9 C3 TP/l E m f v 7 1an and the pulse wire lP, and at the end o; t pulse, relay TP will belocked up over lieht contacts TP2. 1Vhen the second tim D n u n pulse isreceived will cause the operation of relay E through V1, TPS, and Bela iE will lock to E? and to (freund. Ple- J: e: lay E closes a circuit lormagnet R1 and when relay T operates as already described, the busysignal is passed back to the penultimate finder. On the release ol thecontrol circuit of the penultimate linder the circuit for relay A of theinal control circuit is opened and relay A opens the circuit for relay Bwhich closes the restoring circuits of the two magnets R1 and B2. Therestoring circuit for the magnet B1 extends via T2, E3, B2, and theott-normal bank 6, whilst that for the magnet B2 extends via theoil-normal bank N2 and B5. It the num-- ber dialled is one of a P. B. X.but not the first line, automatic limiting will not take place due tothe fact that the battery potential is only connected to the irst lineof the group. The cross connection between the bank C ot' the switch R2and the bank 9 ot the switch R1 is such that consecutively numberedlines may both be P. B.. X. groups oi' more than one line due tothe factthat the switch R2 steps round until it reaches 'he position marked bythe switch R1.

Common spark-quenching condenser and resistances for the driving magnetsS of the conversation switches at each numerical selecting stage arelocated in the controlv circuits. y

The operation of the modified selector circuit illustrated in Figure 6will now bev described. As illustrated this. selector circuit can besubstituted in the system previously described in place of the selectorcircuit shown in Figure 3.

The selector circuit with its associated control switch is testedthrough thev wire T, contacts HSl, TTS, TTG, P2, Q1, E9, the wiper andhome contact of the bank 10 of the switch R1, home Contact and wiper ACof the switch B2, B7, the home contact and wiper of the bank BC of theswitch R3, B4, to resistance and battery. If this resistance and batteryis connected to the test wire the previous control switch will be ableto switch through in the manner described in the call already tollowedout, extending the subscribers loop to BP+ and BP- thereby causing theoperation of relay A through H6 and H7. Relay A closes the obviouscircuit for relay B through A2, and relay B places ground on the testwire at B1 and removes battery from the test wire at B7 and B4. At B6 acircuit is closed for relay M from battery and winding of relay M, D9,H13, B6, wiper and home contact of the bank AC, home contact and wiperof the bank 10, E9, Q1, P2, TTG, B1 to ground. At the first impulserelay A releases but relay B being of the slow release type remainsoperated and completes the circuit for the magnet B1 through E1, B3,winding of relay C, A1 to ground. At the end of the impulse relay Areoperates and the switch R1 steps into position 2. Relay C is also ofthe slow release type and remains operated during the series ofimpulses. Relay C also closes the following circuit for the magnet R2,through its interrupter springs, D4, the wiper and home Contact ofthebank N2, C2, Q1, P2,

relay C will release after TT6, B1, to ground. The switch R2 opens itsown circuit when it steps off normal. When the switch R1 steps intoposition 2 at the end of the first impulse the circuit for relay M isopened by the wiper l0 and when relay M releases the circuit for themagnet R2 is again completed through the oii normal contact of the bankM2, M2, H3, E4 to ground. The switch R2 continues stepping by means ofits own interrupter springs until relay M reoperates. The switch R1takes one step for each impulse received from the subscribers dial andthe strapping between the banks 10 and AC is so arranged that thecircuit for relay M is completed whenever the switch R2 reaches thecommencement of the group corresponding to that on which the switch R1is standing. The switch R2 has access to 100 outlets which will serve a.numer of groups which are themselves dii'erentiated by the setting ofthe switch R1 in response to the dialled impulses. Other groups arereached by the banks ot the switch R3 and a discriminating relay D isprovided to operate when the switch R1 reaches the iirst group connectedto the switch R3. This circuit may be traced from battery, winding ofrelay D, D5, wiper and bank Contact 10 of the switch R1, E9, Q1, P2,TT6, B1 to ground. Relay D locks through D6 to ground at B1. It relay Dis caused to operate the circuit for relay M is transferred through D10,H15, and B5 to the bank BC of the switch R3 and the commoning betweenthe bank 10 of the switch R1 and the bank BC is so arranged that thecircuit tor relay M is completed whenever the switch R3 reaches thecommencement of the group corresponding to that in which the switch R1is standing. The driving circuit of the switch R3 extends over theinterrupter contacts, D3, off-normal bank N2, M2, H3, E4, and after theswitch has moved a step, over its oil-normal bank N3. When the digit iscompleted a period and when relay M operates due to the switch R2 or R3having reached the position indicated by the bank 10, a circuit will becompleted for relay E from battery and winding of rela-y E, C1, M1, H3,E4, to ground. Relay E will close its light contact E5 and lock over itssecond winding to ground at B1. E9 opens the circuit for relay M whichreleases and recloses the driving circuit of the switch R2 or R3 throughM2, H3, E3, Q1, P2, TT6, to ground at B1. Contacts E7 and E8 close thetesting circuit for relays Q and P respectively and one or both of theserelays will operate if the outlets on which the wipers are resting arefree. If relay P alone operates, the driving circuit of the switch willbe opened at P2, at P1 a circuit will be closed for relay H which willlock up through H1, and TTG', to ground at B1. It relay Q operates, thedriving circuit of the switch will be opened at Q1, at Q2 a circuit willbe closed for relay J which will operate and close at J 1 a circuit forrelay H which will also operate. It all the outlets of the group areengaged the switch R2 or R3 will step eventually to the last contacts otthe group in which position a circuit will be closed for relay M throughD9 or D10, H13 or H15, B6 or B5, wiper and bank Contact AC or BC, E10,Q1, P2, TTG, B1 to ground. Relay M will open the driving circuit of theswitch at M2 and close at M1 a circuit for relay G. Relay G is of theslow to operate type and will not operate during the period when relay Mis operated and relay C normal, during the initial operation of theformer. However, at the last Contact of the group the circuit for relayG is maintained and this relay will eventually operate and connect at G3the busy tone to the calling subscriber over lead BT. /Vhen relay Eoperated a circuit was completed for the switch R1 through E2,interrupter springs T1 to ground. This will cause the switch R1 tocontinue stepping until the relay T operates through the wiper 3,contacts HS2 and the hold wire H to the preceding' selector. It will beunderstood that there is only one idle selector accessible to thiscontrol switch with a ground potential on the H wire at this time. Whenrelay T operates the obvious circuit is closed for the relief relay TT.TTl and TT2 extends the subscribers loop through relay A over the wipersand contacts of banks 1 and 2 of the switch R1 and thereby replace thebye-path. TT removes the ground from the test wire causing the releaseof the previous Vcontrol switch; TT4 provides a locking circuit for therel( y T and extends ground to the hold wire H to hold \'the switchingrelays of the preceding switches. When both the relays TT and H areoperated due to the fact that the switch R2 and R3 has found a treeoutlet in the required group and the switch R1 has found the selectorwhich it had been pre` determined will complete the call, a circuit iscompleted for the selector magnet SB via H4, TT7, F2 and bank 7 of theswitch R1 and if the relay D is not operated, for theV selector magnetSA also via D2, M4 and the bank 6. It the discriminating relay D hasbeen operated, the bye-path has been established by the switch R3 andthe connection will be established by the switch SB of the selector.

In this case the magnet SB will cause itsl wipers *B, +B, HB and MB tostep until the following circuit is established :battery and winding ofrelay F, D18, H16, J6 or J7, wiper and bank Contact of the bank BM2 orJM1, multiple wire to the contact and wiper MB of the selector, bankcontact and wiper of bank l0 of the switch R1, TTS, Q1, P2, TT-toground. Relay F in operating completes the circuit for relay HS via F1,D1, TT7, H4.

lib..

` ing control switch under It the' discriminating relay D was notoperated both thev magnets SA` and SB would cause theirl wipers torotate. The circuit for the magnet SB will be opened by the operation ofrelay F which is connected through D17 to a certain predetermined bankcontact of the bank MB of the switch B6 and the circuit torthe magnet SAwill be opened by the operation ot rela-y M through D9, H14, Jl or J5,the wiper and bank contact of the bank AM1 or AM2, multiple wiring tothe bank contact and wiper ot bank MA of the selector, bank contact andwiper 9 ot the switch R1, TTS, Q1, P2, T5, to ground. The wipers of theswitch A6. may be connected to two,

three or more predetermined positions ot the switch B to reduce thehunting time ot this switch: in this case corresponding contacts inthebank MB will be connected to-the relay F. lllhen both the relays M and Fare o*l erated the circuit is completed tor the relay HS through bankcontact and wiper ot bank 4 of the switch R1, F1, M3, D2, TTT, Hl toground. Relay HS in operating will extend ground over the hold wire fromHS3 to the preceding selector and extend ground over HSG to thev holdwire of the succeeding selector. Contacts HSl and HS5 will prepare thepermanent circuit to replace the bye-path.

E lit the circuit is being extended by the switch AG- of the selectorthev conductors will be extended through the wipers and a predeterminedbank contact set of the switch B6. 1t will be seen, that when the twosets ot wipers are rotating simultaneously there is no possibility oftwo sets ot outlets being connected together by the wipers. Furthermore,it the call is being extended by the switch B6 the position of theswitch A6 is immaterial and this switch may therefore be lett in anypositionin which it may be standing. The control switch does not releaseuntil the ground potential over the test wire is removed by thesucceeding control switch. l/Vhen the relay H operated the circuit forrelay A was opened and relay B6 released also atter a sho-rt period.Relays E and T were maintained, however, by the ground potential on thetest wire which is applied by the succeedthe control of the callingsubscribers loop. Then the succeeding selector removed ground on thetest wire relays T and E release and subsequently relays TT, M and Hrelease. When relay H releases the homing circuit for the switch R1 iscompleted through El, B2, H2, off normal contact and wiper of the bankN1, interrupter springs, to ground at T1. Tn a similar way the homingcircuit Jfor the switch R2 is completed tl` Tough D4,the ott normalcontacts N2, M2, H3, Ell to ground and the circuit for the switch R3Vthrough the off normal contacts` N3, M2, H3, E4 to ground. lNhen boththese switches have returned to their home positions the battery isconnected to the test wire- The magnets R1, R2 and R3 are each provided.with their own spark quenching combination consisting of a condenserand.

resistance is provided for the magnets SB :v

these are associated over the banks 5 and 8 of the switch R1.

It will be understood that this arrangement is not limited to twopermanently associated switches and that if the number of groups ofoutlets is suiiicient a further switch Re may be added to the controlcircuit which will operate under the control ot a further discriminatingrelay similar to D and a further switch (c) will be added to theselector circuit and connected through a predetermined position orpositions on the bank of the switch A.

Reference will now be made to Fig. 7 which shows a. method whereby agroup of conversation switches may be switched trom a faulty controlcircuit to an adjacent control circuit which normally serves a differentgroup of conversation finder switches.' The figure illustratessuiiicient ot the circuit of two control circuits C1 and C2 and certainot their associated conversational finder switches for a completeunderstanding oi the arrangement. Two conversation finders 1 and 2 otthe group ot, say ten conversation finders associated with the controlcircuit C1, and one conversation finder ot the group associated with thecontrol circuit C2, are shown. B131 represents the wipers of thebye-passing switch associated with the control circuit Cl, and BP2 thewipers ot' the bye-passing switch associated with the control circuitC2. Each conversation linder is provided with an attaching relay KSl,KS2, or KS2() holding relay HSl, HS2, or HS20, and stepping magnet S1,S2 or S20. Three multiple wires extending from the control circuits andadapted to be operatively associated with the talking group iinder overcontacts ot' the KS relays are shown.

As already fully explained it is the function of a control circuit toreceive the dialled impulses and cause its bye-pass switch to select anidle or wanted line. The relay H (or H1) is then operated and theconnection extended to the wipers ot this switch, as already described.Relay H (or H1) applied a marking potential to a marking contact of theselected trunk over the4 winding or test relay M (or M1) and frontcontacts 1 of relay H (or H1) connects an opposite potential to thecontrol wiper M of the conversation finder switch over front contacts 3ot relay H (or H1), back contacts of a relay @Q1 (or @2Q-2) and frontcontacts of relay KSl, (or KS2' or KS20), and closes a steppingz circuitfor the magnet SL (or S2 or S20) 4terraces of the conversation finderover back contacts of relay M (or M1), front contacts 2 of relay H CSorH1), back contacts of relay QQl (or Q 2) and front contacts of relay KSl(or KS2 or KS20). l/Vhen the conversation iinder switch reaches themarked trunk, test relay M (or Ml) energizes and opens the steppingcircuit of the S magnet. 'Ihe Vcontrol circuit is detached subsequentlyas previously described.

The control circuits are paired and arrangements are made whereby eachcontrol circuit of a pair acts as a reserve for the other. For thispurpose each control circuit Cl or C2 is provided with a change-overrelay QQI or QQ2 which when energized switches over the multiple wiresfrom its own control circuit to the other. rlhe circuit of this relayQQI or QQ2 may be completed manually by throwing a change-over key QK orthe relay may be automatically energized by causing a faulty relay (notshown) to earth the wire F.

It will be apparent that if mutual operation is to be arranged betweenthe two control circuits, the marking contacts of the byepass must bemultipled over all the conversation finders of the two groups, and toensure that there will be no confusion when both control circuits areoperative, the marking condition must be such that the test relay M orM1 of one control circuit will not operate due to a marking potentialfrom the other control circuit. On this account, relay M is connected tobattery and the associated selectors search for a battery potential,whereas relay M1 is connected to ground and the associated conversationlinder switches search for an earth potential.

As both bye-pass switching cannot seize the same trunk there is nodanger of both otentials being placed on the same terminal. For the samereason there is no possibility of the two test relays M and Ml beingenergized in series.

As an alternative arrangement the test relays M, M1 may be placed in thecontrol circuit in the leads which are extended to the conversationiinder switch marking wipers. In this case as it is possible for bothconversation finders switch wipers to rest on contacts belonging to thesame outlet the test relays must be designed not to operate in series.

To apply this feature to the circuits shown in Fig. 2 for example, thewipers of the switch Rl are arranged to give access to conversationfinders of two groups, viz., those of the group to which the finderillustrated belongs, and in later positions to finder ot an adjacentgroup normally served by another control circuit. The wiper 2, Fig. 2searches over the positions allotted to that group to which the findershown belongs, but if the control circuit normally serving the othergroups becomes faulty a relay -such as QQl, Fig. additional wiper l(over contacts QQ) which is arranged to search over the finders of thisother group. In this manner the control circuit shown is made to servethe two groups of iinders.

I claim:

l. In a telephone system, the combinationl; of a plurality ofconversation switches, a con-A trol switch, means for operating saidcontrol switch in response to impulses to hunt for a predeterminedconversation switch and for marking said switch.

2. In a telephone system, a plurality of switching stages, a pluralityof conversation switches at each switching stage, control circuitsassociated with said plurality of conversation switches at eachswitching stage, a byepass circuit extending from one switching stage toanother, and means under control'of said control circuits at oneswitching stage whereby an idle conversation switch and a controlcircuit at the second switching stage are taken in use oversaidvbye-pass circuit extending from a control circuit which has beenselected for use with a conversation switch at the preceding switchingstage.

3. In a telephone system, the combination of a plurality of switchingstages, conversa,-V tion switches at each switching stage, controlcircuits, associated with conversation switches at each switching stage,a test conductor for testing a conversation switch associated with anidle control circuit at one switching stage, and means for subsequentlytransmitting a signal back over said test conductor from the succeedingswitching stage. Y

. el.. In a telephone exchange system, the combination of an vautomaticswitch having a group of outlets, an auxiliary switch of thesinglevmotion type which responds to digit impulses, means fortransmitting such impulses to said auxiliary switch, means according tothe value of the first digit received by said auxiliary switch to causethe advance of said automatic switch to a wanted group of outlets or tocontrol the advance of the auxiliary switch to a predeterminedintermediate position, means in response to a second digit impulse tothereafter advance the automatic switch to a group of outlets determinedby the second digit.

5. In a telephone exchange system, two permanently associated switchesprovided with wipers, the wipers .of one switch being directly connectedto wipers of the other and the wipers of the other connected to aparticular set of bank contacts of said first switch, and means providedfor advancing said iirst switch alone or for simultaneously advancingboth of said switches.

6. A telephone system comprising a plurality of switching stages, eachstage having a plurality of conversation switches, control 7, connectsto the wiper 2 an

